The present invention relates to the purification of low grade acetonitrile to produce higher grades of acetonitrile.
There are currently five types of acetonitrile commonly used in the marketplace: raw acetonitrile, industrial grade acetonitrile, High Performance Liquid Chromatography (HPLC) grade acetonitrile, DNA synthesis grade acetonitrile, and ultra-pure acetonitrile. There are three sources of low grade acetonitrile not yet being utilized.
Raw acetonitrile typically contains up to 50 weight percent acetonitrile and up to 50 weight percent water and is derived from a side reaction during the acrylonitrile manufacturing process. This raw acetonitrile is then purified to meet industry quality specifications for use. Raw acetonitrile is the only source today for making the higher grades of acetonitrile.
Industrial grade acetonitrile is at least 99.75 percent by weight acetonitrile, contains approximately 500 ppm of water and is typically used in gas chromatography applications and agricultural pesticide manufacturing processes.
HPLC grade acetonitrile is a high purity acetonitrile containing at least 99.9 weight percent acetonitrile, a U.V. absorption spectrum measurement of less than 1 angstrom at a 190 nanometer wavelength, and can contain more than 100 ppm of water. HPLC grade acetonitrile is typically used to purify and measure synthetic molecules and DNA probes.
DNA synthesis grade acetonitrile has a purity of at least 99.9 percent by weight acetonitrile, contains approximately 50 ppm or less of water, and has an ultraviolet absorption spectrum measurement of less than 1 angstrom at a 190 nanometer wavelength and zero angstroms at a 260 nanometer wavelength. DNA synthesis grade acetonitrile is typically used as a washing agent, reaction solvent, and a diluent in the DNA synthesis process. DNA synthesis grade acetonitrile is also used in the manufacture of DNA synthesis chemicals.
Ultra-pure acetonitrile has a purity of at least 99.99 weight percent acetonitrile, contains approximately 20 ppm or less of water, and has an ultraviolet absorption spectrum measurement of less than 1 angstrom at a 190 nanometer wavelength. Ultra-high purity acetonitrile is typically used in pharmaceutical drug manufacturing processes.
Low grade acetonitrile is primarily derived from DNA synthesis process waste, HPLC process waste, and pharmaceutical drug manufacturing process waste. These wastes are currently being disposed of by fuel blending (reclamation) or incineration. Low grade acetonitrile is primarily from 30 to 85 percent by weight acetonitrile and contains less than approximately 40 percent water.
DNA synthesis processes typically generate three types of hazardous waste: non-halogenated acetonitrile waste, halogenated dichloromethane waste and aqueous waste. The non-halogenated acetonitrile waste generally consists of a number of constituents which include, but are not limited to: acetonitrile (ACN), dichloromethane (DCM), tetrahydrofuran (THF), pyridine, n-butanol, acetic acid, isobutyl acetate, acetic anhydride, water and other trace constituents including salts and other high molecular fragments.
A number of methods have been developed to purify acetonitrile and various other solvents from raw, industrial grade or higher grade feedstocks. However, there has not been developed an efficient process for purifying acetonitrile from low grade acetonitrile feedstocks.
Prior methods of purifying acetonitrile from raw acetonitrile feedstocks rely on azeotropic or extractive distillation. The method employed is typically dependent on the type and amounts of impurity constituents, including water, the ultimate product purity required, and method economics. Extractive and azeotropic techniques require a separating agent to entrain or extract acetonitrile from its impurities during equilibrium contacting to also obviate having to deal with azeotropes. Acetonitrile is then separated from this added component. For example, U.S. Pat. No. 2,807,573 discloses the purification of acrylonitrile from impure mixtures containing acetonitrile using extractive distillation with water as the separating agent. These processes require relatively large and expensive equipment and have relatively high energy requirements. They also incur additional environmental impact with the handling, loss and disposal of at least one or more additional solvents.
Prior methods of purifying acetonitrile from industrial or higher grade acetonitrile feedstocks include treatment by various absorbents to remove organic impurities (e.g., active alumina, active bauxite, active carbon, special aluminosilicates, molecular sieves, treated clays, Fuller""s earth, diatomaceous earth) and to remove water (e.g., active alumina, calcium chloride desiccants, silica gel, aluminosilicates, molecular sieves and other inorganics, including their oxides and carbonates). For example, U.S. Pat. No. 2,107,904 discloses absorbents for absorbing nitriles from liquid hydrocarbon mixtures using alumina and other absorbents. U.S. Pat. No. 2,560,931 discloses the dehydration of acetonitrile by treatment with activated alumina.
Other methods of purifying acetonitrile from industrial or higher grades of acetonitrile have included the use of oxidizing agents such as air, oxygen or ozone, catalytically or not, followed by distillation and/or absorption to remove the oxidized impurities. U.S. Pat. No. 5,426,208 discloses a method of using ozone to oxidize the deleterious impurities of acetonitrile to produce a purified acetonitrile. One problem with this method is that it requires a feedstock that is essentially 99.95 percent acetonitrile.
Fractional crystallization can also be used to purify acetonitrile. However, the use of a fractional crystallization process requires an initial feedstock composition of 99.9+ percent acetonitrile. The fractional crystallization process is also complicated and expensive in terms of initial investment, energy requirements and maintenance.
A method and apparatus for purifying acetonitrile and other constituents within low grade acetonitrile feedstock is disclosed. In accordance with one embodiment of the invention a process for purifying an acetonitrile feedstock involves purifying a low grade acetonitrile feedstock comprising acetonitrile, a first set of impurities having a lower boiling temperature than acetonitrile and a second set of impurities having a boiling temperature greater than acetonitrile, the process comprising the steps of: a) introducing the feedstock into a first distillation column and separating the acetonitrile and first set of impurities from the second set of impurities, the acetonitrile and first set of impurities being drawn as a vapor from said first distillation column, the second set of impurities being produced as the first distillation column bottoms; b) condensing the vapor to produce a first distillate; and c) introducing the first distillate into a second distillation column and separating the first set of impurities from the acetonitrile, the acetonitrile being produced as the second distillation column bottoms.
In another embodiment, the acetonitrile feedstock includes dichloromethane and tetrahydrofuran. The acetonitrile is purified in the manner described above. The dichloromethane is also purified by directing the vapor from the first distillation column through a first heat exchanger where the temperature is lowered below the dew point of the acetonitrile and tetrahydrofuran. The acetonitrile and tetrahydrofuran are substantially condensed while the dichloromethane vapor is directed to a second heat exchanger where it is condensed.
In yet another embodiment, a DNA synthesizer is provided having its waste effluent directly coupled to an acetonitrile purifier. In one embodiment, the DNA synthesis waste is segregated into acetonitrile waste, dichloromethane waste and detrilylation waste. The acetonitrile waste is coupled to the purifier which includes a first and second distillation column as described above.
In another embodiment, a high performance liquid chromatography instrument is provided having its waste effluent directly coupled to an acetonitrile purifier. The purifier includes a first and second distillation column as described above.
The process of the present invention removes impurities from inexpensive low grade acetonitrile feedstocks to produce industrial, HPLC, DNA synthesis and ultra-pure grades of acetronitrile.
Other features and advantages of the invention will be apparent from the accompanying drawings and and from the detailed description that follows below.